Young people’s sudden knee pain afraid to bend may be meniscus injury, rheumatoid arthritis, knee synovitis and other diseases. 1. Meniscus injury: the occurrence of this disease is related to indirect violence trauma such as knee twisting, followed by chronic injury caused by long-term weight bearing. Commonly high incidence of people are sports enthusiasts, athletes, etc. Typical symptoms are pain, swelling, activity limitation, which can be manifested as a sudden pain in the knee dare not bend. 2. Rheumatoid arthritis: the cause of the disease is still unclear, may be related to autoimmunity, heredity, infection, smoking, pathogenic microbial infection, humid environment and other reasons. The patient’s knee joint will be swollen, painful and other symptoms, but also affect the activities. 3. Knee synovitis: the occurrence of this disease may be related to knee injuries, degenerative changes in the knee joint, can be induced by obesity, long-term physical labor, trauma and other factors, and there will be swelling, quadriceps atrophy and other symptoms. The causes of young people’s sudden knee pain afraid to bend more reasons, need to go to the hospital, clear specific causes, targeted treatment.